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Wind Power That Floats

Advances in floating platforms could take wind farms far from coasts, reducing costs and skirting controversy.

Offshore wind-farm developers would love to build in deep water more than 32 kilometers from shore, where stronger and steadier winds prevail and complaints about marred scenery are less likely. But building foundations to support wind turbines in water deeper than 20 meters is prohibitively expensive. Now, technology developers are stepping up work in floating turbines to make such farms feasible.

Ocean bound: Since December, this prototype 80-kilowatt floating wind turbine has been absorbing wind energy off Puglia, Italy, in 108-meter-deep water, beyond the economically viable depth for turbines mounted on the seabed. Data from the machine’s interactions with wind and waves will inform the final design and control schemes for full-size floating turbines.

Several companies are on their way to demonstrating systems by borrowing heavily from oil and gas offshore platform technology. In December, the Dutch floating-turbine developer Blue H Technologies launched a test platform off Italy’s southern coast; last month, the company announced its plans to install an additional test turbine off the coast of Massachusetts, and possibly begin constructing a full wind farm off the Italian coast, next year. Close behind is SWAY, based in Bergen, Norway, which raised $29 million last fall and plans to field a prototype of its floating wind turbine in 2010.

If these efforts succeed, they could open up a resource of immense scale. For example, according to a 2006 analysis by the U.S. Department of Energy, General Electric, and the Massachusetts Technology Collaborative, offshore wind resources on the Atlantic and Pacific coasts exceed the current electricity generation of the entire U.S. power industry.

The success of the floating turbine could hold the key to exploiting that resource. Wind farms such as those installed in Denmark, Germany, and other European waters and proposed for Nantucket Sound, in Massachusetts, suffer from a limited supply of marine construction equipment such as pile drivers and cranes. Emerging Energy Research, a consultancy based in Cambridge, MA, said last week that the global market for offshore wind energy could reach 40,000 megawatts by 2020–enough to power more than 30 million U.S. homes, and more than twice the scale of last year’s wind installations worldwide–but only with greatly expanded marine construction capacity. Building even 2,000 megawatts of offshore wind over the next five years will require a significant increase in the marine supply chain, according to Keith Hays, the consultancy’s research director.

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Floating turbines can be assembled onshore and towed into position, making an end run around the offshore construction bottleneck. The platform that Blue H towed out of Brindisi Harbor in Puglia, Italy, this winter is called a tension-leg platform, a conventional offshore oil and gas platform design that floats below the surface, held rigidly in place by chains running to steel or concrete anchors on the seabed. Installed on top is an 80-kilowatt wind turbine fitted out with sensors to record the wave and wind forces experienced 10 kilometers offshore. Much bigger floating versions–2.5-megawatt and 3.5-megawatt turbines of the scale used in today’s offshore wind farms–are under construction by Blue H and could be installed as soon as this fall.

What’s unusual about Blue H’s design is the turbine’s two-bladed rotor–a design that lost out to the three-blade design in the 1990s as the wind-turbine industry scaled up. Martin Jakubowski, Blue H cofounder and chief technology officer, says that the noise and jarringly high rotation speeds that made two-bladers a loser on land are either irrelevant or a plus offshore. Faster rotation, meanwhile, offers two benefits. Jakubowski says that the 30-to-35-revolutions-per-minute frequency, twice that of a three-bladed turbine, is less susceptible to interference from the back-and-forth swing of the platform under wave action.

Turbine structure: The steel superstructure below this wind turbine is called a tension-leg platform, similar to what’s used in offshore oil and gas platforms. Once positioned offshore, the platform is held rigidly in place by chains running to a steel and concrete counterweight on the seabed.

Faster rotation also means less torque, meaning that the entire structure can be built lighter. (See “Wind Power for Pennies.”) The rotor, gearbox, and generator of Blue H’s 2.5-megawatt turbine will weigh 97 tons–53 tons lighter than the lightest machine of the same power output on the market. “This is a big advantage,” says Jakubowski. “For us, weight on top is something we have to push up.” The turbine and platform are correspondingly cheaper to build, he says. The net result, says Jakubowski, should be a highly competitive energy source. He estimates that Blue H’s wind farms will deliver wind energy for seven to eight cents per kilowatt-hour, roughly matching the current cost of natural gas-fired generation and conventional onshore wind energy.

And it will be out of sight and thus, the company hopes, out of mind for competing local interests such as tourism. The site off Cape Cod where Blue H intends to install a test platform next summer for its first U.S. wind farm will be 23 miles off the coast.

Blue H’s Norwegian competitor SWAY is using a different combination of offshore platform technology and turbine design. SWAY’s platform is, in essence, a spar buoy that can rise and fall gently with wave action, thus requiring less anchoring than the tension-leg platform. The buoy, a column nearly 200 meters tall, will be held in place by a 2,400-ton gravel ballast on the seabed. Its turbine is three-bladed, but in contrast to conventional onshore turbines, it is allowed to face downwind rather than held upwind to better accommodate heeling of the tower.

Paul Sclavounos, a mechanical engineer and a specialist in naval architecture at MIT, whose lab is designing both kinds of structures for offshore turbines, says that both companies have chosen viable flotation methods, although he believes that the spar approach taken by SWAY will be better adapted to rougher waters. He says that Blue H’s platform may work off the Italian coast, but anchoring it to handle the 30-to-40-meter waves that New England’s storms can whip up may not be economical. “The cost that really drives this business is primarily the foundation,” says Sclavounos.

Where he questions both firms is in their decision to redesign the wind turbines. Sclavounos says that his group is designing both spars and platforms to carry conventional five-megawatt turbines designed for onshore or shallow-water offshore applications. “You don’t want to redesign the turbines for offshore deployment because that’s going to be very expensive, and it’s probably not necessary early on,” he says.

In Sclavounos view, the economics of the power industry are already approaching a tipping point that will drive rapid adoption of floating turbines. “The technology is essentially proven,” he says. “We know we can design [platforms] and spars that are not going to move in big storms. What is going to lead to this industry taking off will be the economics. When carbon-emissions trading markets start maturing, you’re going to see this industry take off, even without state subsidies. We’re not far from it.”

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